Presently, in order to record the positions reached by a moving part (such as the multiple axis, moving arm of an industrial robot) it is common to use an angular-position transducer associated with the motor that moves the part. The transducer, usually consisting of a resolver, generates a signal indicating the angular position reached by the motor during a rotation. Since a typical industrial robot includes several axes of movement, it is desirable to use several resolvers so that movement may be monitored for each axis. The signal generated by the resolver associated with each axis of movement of the robot arm is transmitted to a general control unit (or servo controller--SC) which accumulates the signal produced by each resolver (i.e. each axis of movement) to continually provide an absolute indication of the position of the robot arm.
One of the main drawbacks of such a system occurs when the robot is separated from its servo control (for example, in order to transfer it from the production premises to the user location or from one user location to another) or when the servo controller is momentarily powered down (for example, following a power failure). In this instance, the information relating to the position reached by each moving part is lost. In such a case, it may be necessary to completely recalibrate the position tracking system before it may be restarted.
These loss of power situations are complicated by the fact that the robot arm continues to move for a few moments as a result of inertia, until it reaches a final stop position. This final stop position is not recorded by the main position tracking system since the position tracking system in the SC (as well as the robot arm and the resolvers) is no longer receiving operational power. Complicating manners still further, is the possibility that the robot arm may be moved by external forces during transport or by factory workers while powered down.
The effects of the above-mentioned drawbacks may be lessened, at least in part, by using two cascade-connected angular transducers. Two resolvers coupled together via a speed reduction mechanism is an example of such a device.
According to this configuration, the first resolver registers the momentary position reached during a rotation by the motor actuating the moving part, or the second resolver connected to the first resolver via a speed reduction mechanism, such that the second resolver itself performs a single rotation during multiple rotations of the first resolver. Using this configuration, the position of the moving part may be "read" from the two resolvers when the SC is restarted.
This solution, know as a master/vernier or coarse/fine type, is impractical both in terms of space (two resolvers and a speed reduction mechanism must be provided for each axis of movement of the robot) and in terms of cost (two resolvers with the associated precision gearing cost, in fact, more than a single resolver keyed onto the drive shaft).
Another solution is disclosed in U.S. Pat. No. 5,038,272 entitled "DEVICE FOR TRACKING AND RECORDING THE POSITION REACHED BY A MOVING PART, IN PARTICULAR FOR INDUSTRIAL-ROBOT CONTROL, which is hereby incorporated by reference for its teaching on position tracking for industrial robots.
In this system, an auxiliary resolver position tracking system is integrally coupled to the industrial robot and powered by an uninterruptable power supply which includes, for example, a battery. The system monitors the excitation signal applied to the resolvers and the output signals provided by the resolvers to maintain position information for the various axes of rotation. When a failure in the excitation signal is detected, the auxiliary system generates a substitute excitation signal so that it may continues to monitor the output signals of the resolvers.
This system has disadvantages because the battery limits the time during which it may monitor the position information returned by the resolvers. The resolver excitation signals are desirably provided at relatively high current levels and, so, tend to drain the battery rapidly. The size of the battery which may be used in the resolver position tracking system is limited since it is desirable to package the position tracking system with the robot.